The invention is based on a method and an apparatus for determining and controlling the exhaust gas recirculation rate in internal combustion engines. In order to determine the exhaust gas recirculation rate in internal combustion engines, specifically Diesel engines, it is known (from U.S. Pat. No. 4,060,065) to obtain a first datum evaluatable for closed-loop control purposes by means of temperature measurements, which are performed in the inlet area of the Diesel engine and in the final analysis indicate the ratio between the mass of the fresh-air current delivered to the engine and the recirculated exhaust gas current. By means of suitably forming a difference signal, in the known apparatus, a signal used as an actual value is produced at the output of an electronic computer circuit for the exhaust gas recirculation rate, which is linked at a comparison device with a signal which in this case represents the set-point value of the exhaust gas recirculation; the control deviation acts as a trigger variable for adjusting an adjusting member, for instance, a valve, in the exhaust gas recirculation line. The set-point value may be either a predetermined, constant value, such as a comparison voltage, or a signal obtained from specific operating rates of the engine, composed for instance of an rpm signal, an inlet vacuum signal and a generalized engine temperature signal. In the known apparatus, it is disadvantageous that the establishment of an optimal exhaust gas recirculation rate is not even attempted and, if a variable set-point value is used at all, such a value is obtained only quite generally from operating rates of the engine, without more precise data. However, for reasons having to do with ever-increasing pollution and the general scarcity of energy, it is indispensable to operate a Diesel engine as precisely as possible in its general operating data, particularly to keep the exhaust gas recirculation rate within the optimal range as precisely as possible, and naturally at the least possible expense, so that, in particular, it would be possible to compensate for impermissible smoke development, inaccuracies of the engine, possibly incomplete combustion and the like.
It is also known, in order to determine the exhaust gas recirculation rate, to provide mechanically controlled or mechanically regulated systems having measurement of air and fuel quantities; such systems are complicated and expensive and may be subject to increased aging effects.